Why does this volcano appear to spew blue lava?

Footage from the video above, taken by French photographer Olivier Grunewald, showcases some spectacular images of Indonesia's Kawah Ijen volcano. It's the scenes that portray the volcano at night, though, that have set the Internet ablaze. When viewed in the dark, Kawah Ijen appears to spew eerie, beautiful blue lava.

If you haven't seen any of the footage yet, it's worth taking a look. (The video is narrated in French, by the way, but that won't deter you.) You'll find the mesmerizing shots of the ethereal blue lava around the halfway point of the video. The images have not been Photoshopped, filtered or tampered with in any significant way — the volcano really does appear to glow blue.

"The vision of these flames at night is strange and extraordinary," Grunewald told the Smithsonian. "After several nights in the crater, we felt [like we were] really living on another planet."

As Grunewald goes on to explain, however, it's a bit misleading to suggest that the lava itself is glowing blue. It turns out that the electric blue color is not actually gleaming from the lava, but rather from the flaming sulfuric gases that emerge from the volcano along with the lava.

"This blue glow, unusual for a volcano, isn't the lava itself, as unfortunately can be read on many websites," Grunewald explained. "It is due to the combustion of sulfuric gases in contact with air at temperatures above 360 degrees Celsius."

Eruptions at Kawah Ijen include unusually high quantities of sulfuric gas pressured and heated to a temperature occasionally in excess of 600 degrees C. As the gases are exposed to oxygen in the air, the lava ignites them into brilliant blue flames. In fact, there is so much sulfur that it sometimes flows down the rock face as it burns, which is what gives the impression of spilling blue lava. The lava itself actually radiates a red-orange color, much like any other volcano lava around the world.

Grunewald produced these images as part of a documentary meant to illustrate the harsh working conditions that local miners face when exposed to the unusual mixture of gases that spew from the volcano. Miners in the area extract sulfuric rock to supplement their meager incomes (the rock earns them only about 6 cents per kilogram), but they often walk away with chronic health problems such as throat and lung irritation, difficulty breathing and a propensity for lung disease.

While the blue flames can be quite hypnotizing, the gases that cause them are better viewed from a distance. Since miners rarely wear gas masks, they put themselves in great long-term danger when working and breathing amidst the bluish glow.